ข้อตกลงการเรียกใช้: cdecl, stdcall และ System V

For assembly and C to interoperate, both sides must agree on a calling convention: how arguments are passed, who cleans the stack, and which registers must be preserved.

Conventions differ on where arguments go: on the stack, in registers, or a mix. Getting this wrong corrupts data silently.

cdecl (32-bit, the C default) pushes arguments on the stack right to left, and the caller cleans them up after the call.

stdcall also passes on the stack right to left, but the callee cleans the stack via ret n. The Win32 API uses it.

The key contrast:

• cdecl: caller cleans, supports variadic functions
• stdcall: callee cleans, smaller call sites

The 64-bit Linux/macOS convention passes the first six integer args in registers: RDI, RSI, RDX, RCX, R8, R9; extras go on the stack.

64-bit Windows uses a different register set: RCX, RDX, R8, R9 for the first four args, plus 32 bytes of shadow space.

Integer and pointer results come back in EAX (32-bit) or RAX (64-bit); floating point uses XMM0 in 64-bit conventions.

Each convention designates callee-saved registers (the callee must restore them) and caller-saved ones (free to clobber). Violating this corrupts the caller.

System V requires the stack to be 16-byte aligned at a call. Forgetting this crashes code that uses SSE instructions.

Always declare functions on the C side with the matching convention attribute (e.g. __stdcall) so both halves agree.

You learned calling conventions:

• cdecl: stack args, caller cleans, supports varargs
• stdcall: stack args, callee cleans
• System V x64: args in RDI,RSI,RDX,RCX,R8,R9
• Return in EAX/RAX; respect callee-saved registers and alignment